Generally, during operation of a wind turbine, wind impacts the rotor blades and the blades transform wind energy into a mechanical rotational torque that drives a low-speed shaft. The low-speed shaft drives a gearbox that subsequently steps up the low rotational speed of the low-speed shaft to drive a high-speed shaft at an increased rotational speed, wherein the high-speed shaft rotatably drives a generator rotor. In many conventional wind turbine configurations, the generator is electrically coupled to a bi-directional power converter that includes a rotor-side converter (RSC) joined to a line-side converter (LSC) via a regulated DC link. Each of the RSC and the LSC typically includes a bank of pulse width modulated switching devices, for example insulated gate bipolar transistors (IGBT modules). The LSC converts the DC power on the DC link into AC output power that is combined with the power from the generator stator to provide multi-phase power having a frequency maintained substantially at the frequency of the electrical grid bus (e.g. 50 HZ or 60 HZ).
The above system is generally referred to as a doubly-fed induction generator (DFIG) system, whose operating principles include that the rotor windings are connected to the grid via slip rings and the power converter controls rotor current and voltage. Control of rotor voltage and current enables the generator to remain synchronized with the grid frequency while the wind turbine speed varies (e.g., rotor frequency can differ from the grid frequency). Also, the primary source of reactive power from the DFIG system is from the RSC via the generator (generator stator-side reactive power) and the LSC (generator line-side reactive power). Use of the power converter, in particular the RSC, to control the rotor current/voltage makes it is possible to adjust the reactive power (and real power) fed to the grid from the RSC independently of the rotational speed of the generator. In addition, the generator is able to import or export reactive power, which allows the system to support the grid during extreme voltage fluctuations on the grid.
Typically, the amount of reactive power to be supplied by a wind farm to the grid during steady-state and transient states is established by a code requirement dictated by the grid operator, wherein a wind farm controller determines the reactive power demand made on each wind turbine within the wind farm. A local controller at each wind turbine receives and allocates the reactive power demand between the generator sources (e.g., between generator-side reactive power and line-side reactive power).
Integration of wind energy into traditional power grids generally requires that the wind turbines are equipped with “Fault Ride Through” (FRT) capability, such as Low Voltage Ride Through (LVRT) capability that requires the wind turbines to remain connected to the grid for a predetermined time period during grid voltage drops or sags. This requirement is intended to prevent the wind turbines from dropping off of the grid collector bus and causing further grid instability during the fault state. The grid requirements also generally dictate that, during the fault state, the wind turbines supply reactive power to the grid to regulate and stabilize grid voltage as a function of the magnitude of the grid voltage drop. When the fault state is over, the wind turbines must quickly resupply active power to the grid.
For DFIG systems in particular, when a grid fault occurs (e.g., voltage drop), the DFIG needs to supply reactive power to the grid, which results in the strengthening of the grid voltage, which places even more of a burden on the FRT capability of the DFIG system.
To address the above FRT situations, it has been proposed to augment the reactive power capability of the wind farm during an FRT event with reactive power compensation devices. Reference is made, for example, to the paper “Dynamic Reactive Power Compensation During Fault States for Wind Farms with the Consideration of Wind Turbine Protection Effects” presented at the International Conference on Sustainable Power Generation and Supply, China, 2012. This paper discusses the use of reactive power compensation devices, such as a SVC, during grid fault states, such as grid voltage sag. Similarly, the paper “Research on Effects of Wind Turbines Characteristics on Power Grid Stability” presented at the China International Conference on Electricity Distribution (CICED 2014), China, 2014, discusses configuring SVC and SVG devices at the export location of wind turbines to provide reactive power compensation during grid power failures.
US Patent Application Pub. No. 2017/0025858 describes a wind power plant connected to an electrical grid, the power plant including a plurality of wind turbine generators and a Static Synchronous Compensator (STATCOM) device. In a first control mode, the wind turbine generators and STATCOM are operated in master-slave relationship for reactive power generation. Upon a trigger signal, such as a low voltage event on the grid, a second control mode is implemented wherein the wind turbine generators and STATCOM are switched to a slave-master relationship for reactive power generation.
An improved system and method that operate reactive power compensation devices in a wind farm, or with individual wind turbines, in stable and grid fault states would be desirable in the industry.